The diagnosis and treatment of disease often requires a device to view the interior passages of the body or body cavities that may have to be accessed by surgical instruments. The most common way to do this is via endoscopy systems. Endoscopes are well known as devices to relay images of the internal anatomy to the eye of a physician or surgeon. They include flexible endoscopes such as bronchoscopes, gastroscopes, colonoscopes, sigmoidoscopes and others. They also include rigid endoscopes such as arthroscopes, laparoscopes, cystoscopes, uretoscopes and others. Endoscopes may use optical, fiberoptic or electronic devices or systems to relay images to the operator. Endoscopes are typically part of an imaging system. The imaging system usually comprises light sources, cameras, image recording devices and image display devices such as video monitors or printers.
Endoscopes have become smaller and less expensive to build and have resulted in a continuing improvement in image quality. Newer and smaller imaging sensors such as charge coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) image sensors have allowed the cameras to record and transmit a video image to be integrated into the tip of the endoscope.
A problem with integrating these image sensors into the small space available at the tip of an endoscope is that compromises in either image resolution or image dynamic range are usually required. Resolution is the ability to spatially resolve details in an image. Dynamic range refers to range of shades of light and dark that can be captured by the imaging device. A limiting factor for resolution is usually not the optical quality of the endoscope lenses but the number of pixels available on the CCD. A limiting factor for dynamic range is the ability of each pixel of the CCD to capture the light that makes up an image. Smaller image sensors require smaller pixels, and smaller pixels mean less ability to capture a wide range of light levels.
Most endoscopes are equipped with image sensors that can capture a color image when the tissue is illuminated by white light. This is usually accomplished by placing optical filters that transmit different colors over adjacent pixels on the image sensor. Usually these filters are red, green and blue filters, but they may also be other colors such as cyan, yellow and magenta, or other combinations of colors as may be desired. These filters are commonly arranged in a repeating spatial pattern wherein filters of different colors are located over pixels adjacent to one another. A common pattern of red, green and blue pixels is a Bayer pattern. The adjacent color filtered pixels are each assigned the same spatial location in the digital image, even though they are not actually in the same location and thus the features of the image they are measuring are not in the identical spatial location. Usually these pixels are close enough to approximate the optical characteristics of the tissue being imaged, but they may in some cases reduce the ability to accurately locate details, such as networks of blood vessels. In contrast, when the detector's pixels are actually measuring the same location in the image the measurement can be more accurate.
One method of improving the accuracy of imaging can be to use three image sensors maintained at the proximal end of the endoscope. Such sensors split the image into three wavelength components, each with its own image path, so that the images are registered accurately on each image sensor. These types of image sensors are commonly called 3-CCD cameras and are commercially available from companies such as Sony Corporation of Japan. These devices are feasible and produce high quality images when the endoscope relays an optical image outside of the body cavity, rather than transmitting an electronic image, but are costly and cannot be easily implemented in the tip of an endoscope.
Another method of producing high quality images is to use a single monochrome CCD and to sequentially capture images illuminated by different wavelengths of illumination light by changing a filter in front of the sample or target. Such systems have been produced using optical filter wheels as with an endoscope system produced by Pentax Corporation of Japan and have also been produced using liquid crystal color filters or acousto-optic tunable filters placed in front of cameras, such as those available from QImaging Corporation of Vancouver, Canada. While the liquid crystal and acousto-optic filters have good control of exposure time, none are currently available placed at the tip of an endoscope.
Endoscopes with monochrome CCDs have been produced and used in conjunction with rotating filter wheels by Pentax Corporation but these have the disadvantage of fixed exposure duration and fixed relative brightness provided by the filters in the rotating filter wheel.
A more common method of producing endoscopy images has been the integration of matrix filtered CCD or CMOS image sensors in the tip of an endoscope. In order to make the image sensor small enough to fit in the tip of a small endoscope compromises are typically made in the number and size of the pixels available. Pixels are usually reduced to the smallest practical size manufacturable. When the pixels are made smaller, the capacity to capture photons of light is proportionally reduced to loss of the active area of the pixel, and the ability to capture wide ranges of brightness is also reduced. The ability to capture wide ranges of brightness is in part reduced because, in the case of the most common type of image sensor, when the photon is captured in the silicon of the device, it generates electrons which must be stored until they can be read out and measured. The smaller the pixel, the fewer electrons strike it and the fewer it can store, so the more limited the range of brightness that it can measure. If the image projected on the sensor by the endoscope objective varies greatly in brightness, the entire range of information will not be captured and some parts of the image will be too bright while other parts are too dark.
Thus, there has gone unmet a need for endoscopy cameras and endoscopy systems that can improve the performance of endoscopes by improving image qualities such as contrast and dynamic range. The present apparatus and methods provide these and other advantages.